The present invention relates to charged beam irradiation apparatus and more particularly to an apparatus for suppressing electrification of a sample in charged beam irradiation apparatus, which electrification suppressing apparatus suppresses electrification or charge up generated on a surface of the sample under irradiation of a charged beam.
For example, when an electronic circuit is formed by irradiating a charged beam on a semiconductor wafer standing for a sample, a sample surface is charged up by ions implanted in the sample to induce electric charge of opposite polarity to that of the charge up electric charge in the opposite side to the sample surface. As the charge up electric charge grows to exceed a breakdown voltage of the sample, the sample sometimes undergoes dielectric breakdown. Accordingly, for the purpose of neutralizing charge up electric charge generated on the sample surface, charged particles having opposite polarity to that of the charge up electric charge must be irradiated on the sample surface.
JP-A-2-87450 discloses a technique of preventing such charge up. According to the technique, electrons emitted from an electron gun are deflected toward the sample surface so as to be irradiated thereon and charge up on the sample surface built up positively by a positive ion beam is neutralized electrically by the electrons.
However, electrons generated from an electron source such as the electron gun are easy to concentrate at high density but have high energy which is 100 eV or more, with the result that a positively charged up portion can be neutralized sufficiently electrically but because of occurrence of a phenomenon that electrons irradiated on the neighborhood of the charge up portion charge the neighborhood negatively, the entire sample surface cannot be neutralized properly.
Accordingly, electrons used for neutralization of charge up are required to have small energy. Besides, in order to realize sufficient neutralization, electrons are required to exist at high density.
As a method of supplying low energy electrons for the purpose of neutralizing the sample surface, JP-A-61-47048 discloses a method utilizing secondary electrons and JP-A-4-51437 discloses a method in which electrons are decreased in speed to have low energy. JP-A-1-220350 discloses a method in which energy of electrons is lowered especially for the case where a sample is liable to suffer from damage by charge up.
Practically, however, it is very difficult to concentrate low energy electrons at high density because the electrons are caused to diffuse by repulsive action between them.
On the other hand, when a semiconductor wafer coated at its surface with resist is used as a sample, the charged beam irradiation apparatus is in general provided with a cylindrical member called a Faraday cage for measurement of ion beam current which is arranged near the sample to surround the ion beam path.
With the sample surface coated with resist, when an ion beam bombards the sample surface, such a gas as hydrocarbon is generated therefrom and particularly the amount of gas generated is large in the initial phase of bombardment. Structurally, the cylindrical Faraday cage facilitates rapid diffusion of the generated gas and raises a cause of reducing the effect of preventing charge up on the sample surface. More specifically, deficiency of gas near the sample surface leads to deficiency of low energy electrons generated by ionization of the gas under bombardment of ion beam and useful to neutralize the sample surface. Further, part or most of gas rapidly diffusing from the vicinity of the sample is discharged to the outside through an ion beam inlet opening of the cylindrical member and in consequence pressure is abruptly increased at an outside site which is in close proximity to the inlet opening in the initial phase of ion beam bombardment against the sample surface, so that the gas is ionized near the outer side of the opening to generate electrons which in turn combine with positive ions in the ion beam to neutralize particles in the beam. The neutral particles are admitted into the Faraday cage and therefore electrons and ions generated under the bombardment of ion beam cannot be detected by the Faraday cage, resulting in errors in a measured value of ion beam current. Since the ion beam current is a very important value in controlling the amount of ions implanted in the sample, it must be free from errors as far as possible.
Therefore, in order that gas generated from the sample surface can be utilized effectively for prevention of charge up on the sample surface and reduce measurement errors of ion beam current value can be reduced, it is necessary to suppress diffusion of generated gas from the vicinity of the sample surface as far as possible.
Incidentally, since the amount of electrons generated from only the generated gas is deficient to neutralize charge up on the sample surface, there needs a separate apparatus for generating electrons and irradiating the electrons on the sample surface. Namely, by using both the electrons generated owing to ionization of the generated gas and electrons from the electron source, sufficient electrons to neutralize the sample surface can be obtained. However, the apparatus using the generated gas and the electron source in combination in this manner faces the following problems. In the method as disclosed in JP-A-61-47048 which utilizes secondary electrons as an electron source, the amount of emitted secondary electrons depends on the surface condition of a secondary electron emission electrode (dynode) but the surface condition is affected by contamination due to, for example, sputtering by ion beam and disadvantageously becomes very unstable.
Further, the electron source uses a filament and tungsten or the like constituting the filament per se sometimes behaves as a contaminant substance; and besides the life of filament is relatively short, raising a problem in maintenance that the filament must be exchanged periodically.
Further, disadvantageously, the function of monitoring the degree of neutralization of ions caused by the electrons and controlling the supply amount of electrons properly is insufficient.
In addition, there arise problems that the sample is contaminated by plasma and source gas, the ion beam is contaminated by plasma and source gas, and the ion current cannot be measured accurately for a cause of a decrease in the degree of vacuum caused by the source gas.
Especially, as techniques of eliminating the adverse influence due to contaminant substances of high mass, other than electrons, emitted from the electron source, the following prior arts are available.
JP-A-2-54858 describes that in order to concentrate an electron beam for neutralization of ion beam on the neighborhood of a wafer irradiated with an ion beam, an electron beam drawn out of an electron source is deflected by a deflector.
Also, JP-A-3-25846 describes that an electron source is provided at a position remote from an ion beam irradiation area and electrons are transported to the ion beam irradiation area through transport means having a low conductance against gas for generation of electrons.
In JP-A-2-54858, the electron source is masked with the deflector to mitigate the problem of contamination but the necessity of the deflector and a power supply unit for deflection makes the apparatus complicated and large-sized, raising a problem that reliability and cost performance are degraded. Especially, in spite of the fact that contaminant substance does not impinge directly upon the sample surface, the contaminant substance collides with ions in a beam when it flies across the ion beam path and disadvantageously it sometimes reaches the sample.
In the apparatus described in JP-A-3-25846, the remote disposition of the electron source makes the apparatus large-sized and the necessity of the transport means with pressure reduction means makes the apparatus complicated to disadvantageously reduce reliability of the apparatus and raise the cost thereof.